Fix an alpha inheritance bug:

Fix an alpha inheritance bug:

On alpha, PAL is involved in context management and after wiring
the CPU (in alpha_init()) a context switch was performed to tell
PAL about the context. This was bogusly brought over to ia64
where it introduced bugs, because we restored the context from
a mostly uninitialized PCB.

The cleanup constitutes:
o  Remove the unused arguments from ia64_init().
o  Don't return from ia64_init(), but instead call mi_startup()
   directly. This reduces the amount of muckery in assembly and
   also allows for the next bullet:
o  Save our currect context prior to calling mi_startup(). The
   reason for this is that many threads are created from thread0
   by cloning the PCB. By saving our context in the PCB, we have
   something sane to clone. It also ensures that a cloned thread
   that does not alter the context in any way will return to
   the saved context, where we're ready for the eventuality with
   a nice, user unfriendly panic().

The cleanup fixes at least the following bugs:
o  Entering mi_startup() with the RSE in enforced lazy mode.
o  Re-execution of ia64_init() in certain "lab" conditions.

While here, add proper unwind directives to __start() so that
the unwind knows it has reached the bottom of the (call) stack.

Approved by: re@ (blanket)